1. Field of the Invention
Typically, a double overhead camshaft engine is provided with a pair of overhead camshafts for opening and closing intake and exhaust valves, respectively, at a desired valve timing. The overhead camshafts, either one of which is operationally coupled by a drive belt for rotation to a crankshaft, are operationally coupled to each other by a gear mechanism, so as to turn simultaneously.
2. Description of Related Art
A gear mechanism for operationally coupling a pair of overhead camshafts for driving, i.e., opening and closing, intake and exhaust valves includes camshaft gears coaxial with and attached to one end of each of the overhead camshafts. Providing such a gear mechanism allows the overhead camshafts to be located closer to each other, thereby allowing intake and exhaust valves to be located closer to each other and arranged at a small relative angle with respect to a center axis of a cylinder. Because this overhead camshaft mechanism enables an engine body to be constructed so that it is small and compact in size and to be provided with a simple combustion chamber structure, the engine can be improved in fuel combustion efficiency, and hence in fuel economy.
It is widely known to provide the double overhead camshaft engine with a variable valve timing mechanism in order to vary valve timings according to engine operating conditions, such as engine speeds. For instance, in order for the double overhead camshaft engine to have desired engine output properties during idling as well as in ranges of both middle and high engine speeds, it is necessary for valve overlap to be short in time, or small in degree, during idling, and to be longer in time, or larger in degree, at middle and high engine speeds.
To vary valve timing, it is known to provide a variable valve timing mechanism which comprises a camshaft drive pulley, or sprocket, that is driven by a crank pulley, or sprocket, and which is supported, for rotation, by a drive camshaft driven by the crankshaft. The relative angle between the cam pulley and drive camshaft is changed by a pneumatic control mechanism. Such a variable valve timing mechanism is known from, for instance, Japanese Unexamined Utility Model Publication No. 62-57711.
In the variable valve timing mechanism disclosed in the above publication, if helical splines in the variable valve timing mechanism and helical gears for connecting the drive and driven camshafts are used, a large thrust is transferred to the driven camshaft as the valve timing is varied, and the variable valve timing mechanism is prevented from acting smoothly That is, at the beginning or end of a varying operation of the variable valve timing mechanism, a reactive thrust force is imposed on the drive camshaft by the helical gears, and is transmitted to the driven camshaft from the drive camshaft. Accordingly, the driven camshaft undergoes a large lengthwise displacement, due to the transmitted reactive thrust force, in addition to a reactive force produced by torsion produced by the helical gears. If the valve driving system produces a change in driving torque and transmits it to the camshafts, the reactive thrust force increases greatly, and the camshafts adversely affect their mounting elements. A reactive force from the helical gears is transmitted as a thrust to the variable valve timing mechanism, so as to prevent the variable valve timing mechanism from returning smoothly and thereby to increase a delay in operational response.
Furthermore, because of the fact that the drive camshaft is attached with the cam pulley operationally coupled to the crank pulley by a belt, the drive camshaft is, during engine operation, applied with an excess loading by the belt. For this reason, the variable valve timing mechanism, installed at an end of the drive camshaft, is supported in a cantilevered fashion, so as to be possibly structurally unstable. In order for the drive camshaft to resist the loading imposed by the belt, it is preferable that the loading be borne by a portion as close to the variable valve timing mechanism as possible, as well as to have the longest bearing length and the largest bearing surface area possible.
The overhead camshafts, for intake and exhaust valves, must be exactly positioned in a thrust, or lengthwise, direction. More particularly, it is desirable to have the drive camshaft attached with the camshaft drive sprocket positioned, in the lengthwise direction, by a bearing located near the camshaft drive sprocket. This is because a relative displacement is produced in the lengthwise direction between the drive and driven camshafts, due to thermal expansion of the drive camshaft, which causes a displacement of engagement between gears of the drive and driven camshafts, which are in mesh with each other.
For these reasons, the drive camshaft, supported by the bearing near the camshaft drive sprocket and the variable valve timing mechanism, is required to have a high structural strength. Providing the drive camshaft with a sufficient structural strength is accompanied by an increase in length of the bearing bore and, accordingly, an increase in length of the drive camshaft itself. This leads to an increased engine size.